1. Field of the Invention
The present invention relates to a composite magnetic member combining a ferromagnetic portion and a non-magnetic portion suitable for use in an actuator which treats with automobile fuels and hydraulic operating fluids or the like (hereinafter referred to as an oil controlling device).
2. Description of the Related Art
An oil flow controlling device of an automobile conventionally has a structure in which an effective use of magnetic flux is made by providing a non-magnetic portion in a part of a stator, which stator is ferromagnetic (generally, soft magnetism), to cause magnetic flux to flow to a movable piece. Techniques such as the brazing and laser welding of a ferromagnetic part and a non-magnetic part have been employed to provide a non-magnetic portion in a part of the ferromagnetic portion. In contrast to these techniques of bonding dissimilar materials, the present authors propose the use of a single material as a composite magnetic material which is formed by providing a ferromagnetic portion and a non-magnetic portion by cold working or heat treatment. When such composite magnetic materials made of a single material are used, it is possible to obtain parts superior to those obtained by bonding a ferromagnetic portion and a non-magnetic portion with respect to ensuring airtightness and ensuring reliability, such as prevention of breakage by vibrations, etc.
In Japanese Patent Unexamined Publication No. 9-157802 based on a proposal by the present inventors, for example, a martensitic stainless steel containing 0.5 to 4.0% Ni is disclosed as a composite magnetic member suitable for an oil controlling device of an automobile. This proposal is such that in a martensitic stainless steel composed of ferrite and carbides in an annealed condition, the austenite in a non-magnetic portion having a permeability (.mu.) of not more than 2, which portion is obtained by cooling a part of the martensitic stainless steel after heating, is stabilized by adding an appropriate amount of Ni to a C-Cr-Fe-base alloy from which ferromagnetic properties with a maximum permeability (.mu.m) of not less than 200 are obtained, whereby it is possible to lower the Ms point (temperature at which austenite begins to be transformed into martensite) to not more than -30.degree. C.
Also, Japanese Patent Unexamined Publication No. 9-228004 based on a proposal by the present applicant discloses that in a composite magnetic material used in magnetic scales etc., by adding more than 2% but not more than 7% Mn and 0.01 to 0.05% N to a C-Cr-Fe-base alloy containing 10 to 16% Cr and 0.35 to 0.75% C which alloy has ferromagnetic properties with a maximum permeability (.mu.m) of not less than 200, it is possible to stabilize the retained austenite with a permeability (.mu.) of not more than 2, which is obtained by cooling after heating, and to thereby lower the Ms point to not more than -10.degree. C. These proposals are excellent in the respect that a ferromagnetic portion with a maximum permeability (.mu.m) of not less than 200 and a stable non-magnetic portion with a permeability (.mu.) of not more than 2 and a low Ms point can be obtained in a single material.